As life expectancy increases, Alzheimer's disease (AD) is becoming a major health problem in the western world. There has been intensive research aimed at identifying a reliable cure or preventive measures for the disease, so far without success. One of the biggest problems in the design and testing of any therapeutic agent is the lack of clinical diagnostic criteria that could identify AD sufferers early enough for any meaningful intervention. The currently available clinical diagnostic tools do not allow a confident clinical diagnosis of Alzheimer's disease in other than severely demented patients.
Currently, there is no accepted “gold standard” diagnostic test for clinical diagnosis Alzheimer's disease in the live patient. The most often used clinical diagnostic criteria are the NINCDS/ADRDA criteria (McKhann, G. et al., (1984) Neurology 34: 939-944), originally designed for research purposes. These criteria are highly sensitive but have a low specificity. In this context, sensitivity is defined as the probability that the criteria will be satisfied in people who have Alzheimer's disease, and specificity is defined as the probability that the criteria will not be satisfied in people who do not have Alzheimer's disease. As a consequence of their low specificity, the NINCDS/ADRDA criteria are not ideal for clinical diagnostic purposes. Additionally they are not suitable as diagnostic criteria for clinical trials looking at preventive or curative therapies that may have their best chance of being effective if used before significant dementia has developed, since NINCDS/ADRDA require dementia in the patient as a criterion for an AD diagnosis.
A “confirmed” diagnosis of Alzheimer's disease can only be made post-mortem, by histological examination for the characteristic Alzheimer's disease pathology (accumulation of amyloid plaques and tangles), but this approach is clearly of no use for clinical diagnosis of AD in the living subject.
In recent years it has become more widely accepted that a pathogenic basis of Alzheimer's disease is the aberrant re-entry of different neuronal populations into the cell division cycle (Nagy Z, Esiri M M and Smith A D (1998) Neuroscience 84: 731 739). In healthy elderly individuals rapid cell cycle arrest and re-differentiation may follow this cell cycle re-entry. In contrast, in individuals with Alzheimer's disease the regulatory mechanisms appear to fail and the neurons progress into the late stages of the cell cycle leading to the accumulation of AD related pathology and/or neuronal death (Nagy Z, Esiri M M and Smith A D (1998) Neuroscience 84: 731 739). Several studies indicate that the cell cycle regulatory failure in Alzheimer's disease occurs at the G1/S transition checkpoint (see in particular Arendt T, Rodel L, Gartner U and Holzer M (1996) Neuroreport 7: 3047 9).
The appreciation that Alzheimer's disease can result from defective cell cycle regulation at the G1/S transition has led to the development of alternative approaches to AD diagnosis, based on detection of the underlying cell cycle regulatory defect rather than evaluation of outward symptoms of the disease, such as cognitive impairment (dementia). In this regard, International patent publication WO 02/073212 describes a diagnostic test, useful in the diagnosis of early-stage AD, which is based on screening for the presence of a cell cycle regulatory defect at the G1/S transition in non-neuronal cells of a test subject. The authors of WO 02/073212 found that the cell cycle regulatory defect at the G1/S transition previously seen in the neurons of Alzheimer's disease patients also occurs in non-neuronal cells, such as lymphocytes or fibroblasts, of AD patients. This in turn led to the development of a convenient blood test assay for the cell cycle regulatory defect which underlies (and precedes) the development of classic Alzheimer's disease symptoms, such as dementia (Zs Nagy, M Combrinck, M Budge, R McShane. Cell cycle kinesis in lymphocytes in the diagnosis of Alzheimer's disease. Neurosci Letters. 2002, 317, 2, 81-84.).
Apolipoprotein E (apoE) is an apolipoprotein essential for catabolism of triglyceride-rich lipoprotein constituents. The gene encoding apoE is polymorphic, with three major alleles ApoE2, ApoE3 and ApoE4, which translate into three major isoforms of the protein (apoE2, apoE3 and apoE4). The ApoE4 allele is a known genetic risk factor for AD in a variety of ethnic groups and can account for approximately 50% of cases in many populations (Waring S C and Rosenberg R N. Genome-Wide Association Studies in Alzheimer Disease. Arch Neurol 2008; 65(3):329-334). Individuals with either one or two copies of ApoE4 have a higher risk of developing AD, compared with carriers of the other isoforms. ApoE4 also reduces the median age of AD onset from 84 in non-carriers to 68 in homozygotes (Cedazo-Minguez A. Apolipoprotein E and Alzheimer's disease: molecular mechanisms and therapeutic opportunities. J. Cell. Mol. Med. 2007; 11(6): 1227-1238).
Although the ApoE4 allele is an established genetic risk factor for AD, this marker is not useful on its own for the diagnosis of AD in a clinical setting. Furthermore, it has not proven to be useful in combination with neuropsychological assessment of cognitive deficit in the clinical diagnosis of AD in subjects presenting with dementia (McConnell L M, Sanders G D, Owens D K. Evaluation of genetic tests: APOE genotyping for the diagnosis of Alzheimer disease. Genet Test 1999; 3(1):47-53).